The invention relates to a magnetic resonance apparatus, comprising a magnet system which is rotationally symmetrically arranged about a central axis and which comprises a superconducting inner coil system for generating a steady magnetic field in a measurement space within the magnet system, and also comprises an outer coil system which is arranged so as to be coaxial with the inner coil system and which is electrically connected in series therewith in order to shield the environment from the magnetic field generated by the inner coil system, which magnet system can be bridged by a superconducting persistent current switch and comprises a superconducting shunt which bridges a part of the magnet system which is situated between first and second connection points.
An apparatus of this kind is known from EP-B-0 299 325. In the known apparatus the shunt extends between the points where the inner coil system is connected to the outer coil system. The shunt thus allows for different currents to flow in the inner coil system and the outer coil system, so that the effect of external magnetic field variations on the magnetic field at the centre of the measurement zone is reduced. It has been found in practice, however, that in this manner rather incomplete shielding from the effects of external magnetic fields is achieved: in the measurement space a uniform external magnetic field is attenuated to approximately 15% of the original value. Shielding from non-uniform external magnetic fields is even less complete.